Electrical organic thin film switching device switching between detectably different oxidation states

ABSTRACT

A current-controlled, bistable threshold or memory switch comprises a polycrystalline metal-organic semiconductor sandwiched between netallic electrodes. Films of either copper or silver complexed with TNAP, DDQ, TCNE, TCNQ, derivative TCNQ molecules, or other such electron acceptors provide switching between high and low impedance states with combined delay and switching times on the order of 1 nanosecond. Switching behavior of a complex of the present invention is related to the reduction potential of the acceptor molecule.

STATEMENT OF GOVERNMENTAL INTEREST

The invention herein described was made in the course of or under acontract or subcontract thereunder with the Department of the Navy. TheGovernment also has rights in the invention pursuant to Grant No.DMR-76-84238 awarded by the National Science Foundation.

RELATED APPLICATIONS

This is a division of Ser. No. 385,523 filed June 7, 1982, now U.S. Pat.No. 4,507,672, which was a division of Ser. No. 130,400 filed Mar. 14,1980, now U.S. Pat. No. 4,371,883.

TECHNOLOGICAL CONTEXT OF THE INVENTION

In the field of electrical switching, the use of organic thin films hasbeen suggested by the prior and current art. Elsharkawi, A. R. and Kao,K. C. "Switching and Memory Phenomena in Anthracene Thin Films", Phys.Chem. Soc. 1977, Vol. 18, pp. 95-96; Stafeev, V. I., Kuznetsova, V. V.,Molchanov, V. P., Serov, S. S., Pospelov, V. V., Karakushan, E. I.,Airapetyants, S. V., and Gasanov, L. S. "Negative Resistance of VeryThin Organic Films Between Metal Electrodes", Soviet PhysicsSemiconductors, Volume 2, No. 5, November 1965, pp. 642-643; Carchano,H., Lacoste, R., and Segui, Y., "Bistable Electrical Switching inPolymer Thin Films", Applied Physics Letters, Volume 19, Number 10, 15Nov. 1971, pp. 414-415; Kevorkian, J., Labes, M. M., Larson, D. C. andWu, D. C. "Bistable Switching in Organic Thin Films", Discussions of TheFaraday Society, No. 51, 1971; and Szymanski, A., Larson, D. C. andLabes, M. M. "A Temperature-Independent Conducting State in TetraceneThin Film", Applied Physics Letters, Volume 14, Number 3, 1969, pp.88-90 disclose various organic thin film switches. Elsharkawi et alteach an antracene switch; Stafeev et al teach a cholesterol switch; andKevorkian et al, as well as Syzmanski et al, teach "reproduciblebistable switching in aromatic hydrocarbon thin films, such as tetraceneand perylene." The mechanisms of switching in these organic elements arebasically physical in nature. Impedance changes which accompanyswitching are associated with molecular motion due to Joule heatingwhich results in either crystallization phase transitions, metalfilament formation, or elimination of compositional disorder. Switchinghas also been related to electrical breakdown due to weak spots in theswitching material. Switching by means of electrochemical changes is notsuggested by the references. More particularly, switching due toelectrochemical changes in a metal complexed withtetracyanoquinodimethane (TCNQ), tetracyanonaphthoquinodimethane (TNAP),tetracyanoethylene (TCNE), dichlororicyanobenzoquinone (DDQ), or TCNQderivatives is not contemplated. The electrical characteristics ofdevices discussed in the above references may be erratic in nature ornot readily reproducible. Further, these references disclose switchingat impractical voltage or current levels and do not mention high-speed,nanosecond switching. Organic devices in general have not been shown tobe comparable to other inorganic switches currently in use.

Such inorganic devices include switches such as those amorphous alloyswitches discussed in U.S. Pat. Nos. 3,530,441, 3,588,638, 3,644,741,3,715,634, 3,868,651, 3,983,542, and 3,988,720 (by Ovshinsky et al).Amorphous alloys, including chalcogenide glasses, are inorganicsemiconductors which show switching behavior. These glasses contain upto four elements, often including arsenic and/or tellurium, and exhibita current-controlled negative resistance. One such amorphous alloy is

    Te.sub.40 AS.sub.35 Ge.sub.6 Si.sub.18

Other inorganic switching materials include the oxides of nickel,silicon, aluminum, titanium, zirconium, and tantalum, all of whichexhibit a voltge-controlled negative resistance when arranged in ametal-oxide-metal sandwich structure. Switching behavior in theseinorganic materials has been attributed to the forming of filamentswhich are described by both thermal and electronic models. Theelectrical behavior of these materials has generally been found to beunstable, not reproducible, and not independent of polarity of appliedswitching signal. Examining only inorganic materials, theabove-discussed references do not even consider the possibility ofswitching with an organic film of metal complexed with TCNQ, TNAP, TCNE,DDQ, or TCNQ derivatives.

The employment of TCNQ in a switching device has been suggested. Howeverthe phenomena employed are unrelated to electrochemical variations inthe switching material and the materials used are not of a thin filmsemiconductor nature. Devices by Aviram (U.S. Pat. Nos. 3,833,894 and3,953,874) exploit the tunnelling phenomenon in an organic memorycomprising a compound (which may include TCNQ) sandwiched between basemetal layers. The I-V characteristics of the Aviram devices do not shownegative resistance and are not discontinuous at a given threshold. Alsothe role of TCNQ in the Aviram devices is not particularly significant.The switching relates to non-conjugate bridges interposed between tworedox couple elements which, when sufficient potential is applied toovercome a potential well caused by the bridge, interchange structuredue to electron tunnelling. Remaining elements, one of which can be TCNQ(acting as a simple anion), are included to maintain electro-neutralitywhen the two elements interchange configurations. This tunnelling effectworks equally well with various anions other than TCNQ, the TCNQ notbeing essential to the tunnelling. Another patent by Biernat (U.S. Pat.No. 3,119,099) cited in Aviram, Considers changes in molecular structurewhich effect bistable switching. The molecular storage unit in theBiernat patent is described electronically with reference to moleculardistortion resulting in changed energy states.

An article by L. R. Melby, R. J. Harder, W. R. Hertler, W. Mahler, R. E.Benson, and W. E. Mochel entitled "Substituted Quinodimethans. II.Anionradical Derivatives and Complexes of 7,7,8,8Tetracyanodiquinomethane,"Journal of American Chemistry, vol. 84, pp.3374-3387, 1962, describes a method of making cuprous and other metallicsalts of TCNQ. No suggestion is made that any of the salts might existphysically as a film or electrically as a semiconductor or switch.

U.S. Pat. No. 3,331,062 (by Wisdom and Forster) considers the use of ametal electrode immersed in an electron acceptor such astetracyanoethylene (TCNE). The patent discusses the storage ofinformation by means of an electrochemical device, however it is limitedto elements in liquid solution form. The basis for such a device is thechange in capacitance realized when a pulse is applied to a memory cellcomprised of an electrode and a liquid acceptor. While useful, thedevice of U.S. Pat. No. 3,331,062 is not solid-state and requires thecombining of the acceptor with a non-metal donor to form a liquid.High-speed threshold and memory switching by changing resistance statesare not detailed. Combining the teachings of organic and inorganicswitching technology, U.S. Pat. No. 3,719,933 (by Wakabayashi, Kinugasa,Hozumi, and Sugihara) discloses the dispersing of inorganic lead oxidein an organic resin film to achieve a memory device having both a highand a low resistance state.

SUMMARY OF THE INVENTION

In the context of the aforementioned prior and contemporaneous switchingdevices, the present invention is novel and significant by providing amethod of fabricating an organic thin film, solid-state semi-conductor,current-controlled switch comprised of copper or silver complexed withTCNE, TCNQ, a TCNQ derivative, DDQ, or TNAP. Specifically, the presentinvention produces a switch responsive to electrochemical changes in thethin film semiconductor.

It is a purpose of the invention to provide a method of fabricating anorganic thin film device exhibiting switching from a high impedancestate to a low impedance state by applying an electric field of anypolarity. The magnitude of the applied field required to effectswitching depends on the strength of the electron acceptor. Inaccordance with the invention, switching back to the high impedancestate can be achieved by introducing a pulse of polarity opposite thatof the previously applied electric field where the electron acceptor isof relatively intermediate strength.

It is another purpose of the invention to provide a method offabricating an organic thin film device exhibiting reliable, high-speed,reproducible switching in a threshold or memory mode. Whether the deviceoperates in the threshold or memory mode depends on the redox potentialof the electron acceptor which is complexed with the copper or silverand the length and duration of the field applied to generate switching.

The device may, alternatively, operate as a diode if connected to anelectrode of a selected material, such as platinum, gold or magnesium.

It is yet another purpose of the present invention to provide a methodof fabricating an organic, solid-state semiconductor device whichswitches in response a low voltage (e.g. 3 to 12 volts). In the "ON"state, the device will not break down with currents applied in excess of30 milliamperes. These parameters render the present invention practicalfor conventional electronic switching applications.

It is a particularly significant purpose of the present invention toprovide a method of fabricating an organic element which switchesbetween impedance states in the nanosecond range.

It is a further purpose of the invention to provide a method offabricating an organic, solid state semiconductor switch which is, butis not necessarily limited to being, insensitive to moisture, radiation,and temperature. The present switching device is stable at temperaturesbetween -50° C. and 150° C.

It is still a further purpose of the invention to provide a method offabricating a two-terminal device which can be adapted to athree-terminal or other multiterminal switching device or matrix. It isyet a further purpose to provide an organic switching device fabricatedaccording to a process whereby a metal is complexed with TCNE, TCNQ, aTCNQ derivative, or TNAP to form a film. The thickness of the film,generally on the order of 1 to 10 μm, can be carefully controlled.

It is an ancillary purpose of the invention to provide a method offabricating a device which acts as an organic, electrochemical storagecell when passing from a low to a high impedance state.

It is still a particularly significant purpose of the invention tofabricate a thin film to effect switching by means of a field inducedredox reaction in a film comprised of copper (or silver) and TCNQ (TCNE,DDQ, TNAP, or a TCNQ derivative) wherein mixed valence species (i.e.complex salts) are in equilibrium with a simple 1:1 salt. In accordancewith the invention, switching is described by the process equation:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment of the invention;

FIG. 2 shows the I-V characteristics;

FIG. 3 illustrates behavior of different compositions;

FIGS. 4-5 show time dependence of switching.

DESCRIPTION OF THE INVENTION

One embodiment of the present invention is shown in FIG. 1 a 1-10 μmthick polycrystalline film 2 of a copper or silver charge-transfercomplex is sandwiched between two metal electrodes 4 and 6. Oneelectrode 4 comprises a metallic substrate which may be made of themetal found in the charge-transfer complex. The other, a top electrode6, may be selected from various metals, such as aluminum, platinum,gold, magnesium, or chromium. The selection of the top electrode 6,however, may affect the nature of the device 1 shown in FIG. 1.

With the top electrode 6 being either aluminum or chromium, the device 1acts as a switch. Depending on various characteristics (to be discussedlater) of the film 2, the device 1 will act as either a threshold switchor a memory switch. However, selecting a top electrode 6 of anothermetal, such as platinum, gold, or magnesium, can alter the nature of thedevice 1 to function as a diode.

As a threshold switch or memory switch, the device 1 in FIG. 1 is atwo-terminal device which is stable in either a high or low impedancestate. The transition from the high to the low impedance state occurswhen an electric field which exceeds a threshold level is applied acrossthe film 2. The field can be easily generated by providing a voltage,with any of various known means, across the two electrodes 4 and 6through external contact wires 8 and 10, respectively. The upper contactwire 8 is shown connected to the top electrode 6 by a conducting paste(such as silver) or by liquid metals of mercury, gallium, orgallium-indium utectic 12. As a memory switch, the device 1 remains inthe low impedance state after the initial applied field (which exceedsthe threshold) is removed. As a threshold switch, the device 1immediately returns to the high impedance state when the applied fieldfalls below a minimum holding value which, for the present invention, issomewhat below the threshold value.

The behavior of the device 1 as a memory switch or threshold switch isillustrated in FIG. 2. The device 1 displays an S-shaped, negativeresistance I-V characteristic, for a 3.75 μm thick Cu/CuTNAP/Al film 2of a sample device, measured across a 100 ohm resistor. The "OFF", orhigh impedance, state is shown having small current variations forvoltages ranging (in the FIG. 2 sample embodiment) up to 2.7 volts. Atthe 2.7 volt threshold, the device 1 assumes an "ON", or low, impedancestate. Because the I-V characteristic of the device 1 is symmetric aboutthe origin, similar switching occurs regardless of voltage polarity. Asindicated by the dashed load line, the device 1 remains in the "ON"state after the threshold (2.7 volts or -2.7 volts in the sampleembodiment) is exceeded even when the voltage thereafter drops below thethreshold. In the case of the threshold switch, the device 1 returns tothe "OFF" state when the field falls below the holding value. For thememory switch, the device 1 remains in the "ON" state indefinitely inthe presence of an electric field and decays to an "OFF" state over timeif the field is removed. The time required to return to the initialstate appears to be directly proportional to the film thickness, theduration of the applied field, and the amount of power dissipated in thefilm 2 while in the "ON" state.

An interesting characteristic of the present device 1 is that itgenerates a small electromotive force (emf) when it returns to the "OFF"state from the "ON" STATE. In this respect, the device 1 acts as a smallelectrochemical storage cell.

In the sample embodiment of FIG. 2, the threshold field is approximately8.1×10³ V/cm and the high impedance state corresponds to 1.25×10⁴ ohmscompared with 190 ohms for the low impedance state. The time required toswitch back to the initial state appears to be directly proportional tothe film thickness, the duration of the applied field, and the amount ofpower dissipated in the film 2 while in the ON state.

The low impedance state of films comprising copper and silver complexesaccording to the present invention is also related to the acceptormolecule complexed with the copper or silver. For example, copper saltsconsistently exhibit greater stability and reproducibility over thecorresponding silver salts of the same acceptor. Also, preferredswitching behavior of the different complexes is related to thereduction potential of the various acceptors. This is shown in FIG. 3using copper as a donor in each case. For devices made from weakelectron acceptors like CuTCNQ(OMe)₂, the switching behavior is usuallyof the threshold type. That is, when the applied voltage is removed fromthe device 1 which is in the "ON" state, the device 1 will immediatelyreturn to the "OFF" state. On the other hand, for strong electronacceptors, like CuTCNQF₄, a memory effect is observed. This memory stateremains intact from a few minutes up to several days and cannot beremoved by the application of a short pulse of current. For intermediatestrength acceptors, however, the device can operate as either a memoryswitch or a threshold switch by varying the strength or the duration ofthe applied field in the low impedance state. When the device 1comprises an intermediate acceptor, such as TCNQ or TNAP, a return tothe "OFF" state can be achieved by introducing a short high densitycurrent pulse of either polarity.

The required field strength for switching parallels the strength of theacceptor. The copper salt of TCNQ(OMe)₂ switches at a field strength ofapproximately 2×10³ V/cm, while the copper salt of TCNQF₄ is found toswitch at a field strength of about 1×10⁴ V/cm. These effects, shown inFIG. 3, indicate that switching behavior is related to the reductionpotential of the acceptor. In the family of TCNQ derivatives whichinclude, but are not limited to the following:

TCNQ(OMe)

TCNQ(OMe)₂

TCNQ(OMe)(OEt)

TCNQ(OMe)(O-i-Bu)

TCNQ(CN)₂

TCNQ(OMe)(O-i-C₂ H₅)

TCNQ(OEt)(SMe)

TCMQCl

TCNQBr

TCNQClMe

TCNQBrMe

TCNQIMe

TCNQCl

TCNQBr

TCNQI

TCNQ(OMe)(OCH₃)₂

TCNQ(Me)

TCNQ(Et)

TCNQ(i-Pr)₂

TCNQ(i-Pr)

the nature of switching for any given acceptor will be defined as afunction of its reduction potential.

In addition to reduction potential, switching characteristics are alsoaffected by the magnitude and duration of a switching pulse applied tothe device 1. In FIG. 4, the dependence of threshold voltage on pulseduration for an embodiment having a microcrystalline film 2 of coppertetracyanonaphthoquinodimethane (Cur-TNAP) is shown. For a long pulses,the threshold voltage is identical to that shown in the dccharacteristic of FIG. 2. As the pulse length is decreased the thresholdvoltage increases sharply for pulses of 1-5 μsec duration. For pulses ofnanosecond duration, switching still occurs, however, the value of therequired threshold voltage is increased slightly and is less consistentthan at longer pulse durations.

The transient response of the Cu-TNAP embodiment, to a "rectangular"pulse havng a 4 nsec rise time, is shown in FIG. 5. The voltage pulse(shown in the upper portion of the trace) causes the device 1 to switchfrom the high to the low impedance state. The trace in FIG. 5 isparticularly significant, indicating that the mechanism of switching inthe present invention is not of a thermal nature. The mechanism insteadappears to be the result of a field induced solid-state reversibleelectrochemical reaction particularly associated with the metalcharge-transfer salts:

    __________________________________________________________________________    CuTCNQ(OMe)  CuTCNQCl     AgTCNQ(OMe)  AgTCNQCl                               CuTCNQ(OMe).sub.2                                                                          CuTCNQBr     AgTCNQ(OMe).sub.2                                                                          AgTCNQBr                               CuTCNQ(OMe)(OEt)                                                                           CuTCNQI      AgTCNQ(OMe)(OEt)                                                                           AgTCNQI                                CuTCNQ(OMe)(O--I-Pr)                                                                       CuTCNQ(OMe)(OCH.sub.3).sub.2                                                               AgTCNQ(OMe)(O--I-Pr)                                                                       AgTCNQ(OMe)(OCH.sub.3).sub.2           CuTCNQ(OMe)(O--i-Bu)                                                                       CuTCNQ(CN).sub.2                                                                           AgTCNQ(OMe)(O--i-Bu)                                                                       AgTCNQ(CN).sub.2                       CuTCNQ(O--i-C.sub.2 H.sub.5)                                                               CuTCNQ(Me)   AgTCNQ(O--i-C.sub.2 H.sub.5)                                                               AgTCNQ(Me)                             CuTCNQ(OEt)(SMe)                                                                           CuTCNQ(Et)   AgTCNQ(OEt)(SMe)                                                                           AgTCNQ(Et)                             CuTCNQCl     CuTCNQ(i-Pr) AgTCNQCl     AgTCNQ(i-Pr)                           CuTCNQBr     CuTCNQ(i-Pr).sub.2                                                                         AgTCNQBr     AgTCNQ(i-Pr).sub.2                     CuTCNQClMe   CuTNAP       AgTCNQClMe   AgTNAP                                 CuTCNQBrMe   CuTCNE       AgTCNQBrMe   AgTCNE                                 CuTCNQIMe    CuDDQ        AgTCNQIMe    AgDDQ                                  __________________________________________________________________________

Fabrication of the device consists of first removing any oxide layersand organic contaminants from either a piece of copper or silver metal.The cleaned metal is then placed in a solution of dry and degassedacetonitrile which has been saturated with a neutral acceptor molecule,for example, TCNQ. Alternatively, the metal can be immersed in methanolor any other solution which dissolves the acceptor molecule. The neutralacceptors used are preferably recrystallized twice from acetonitrile andthen sublimed preferably under a high vacuum prior to their use. Whenthe solution saturated with the neutral acceptor is brought in contactwith a metal substrate of either copper or silver, a rapidoxidation-reduction reaction occurs in which the corresponding metalsalt of the ion-radical acceptor molecule is formed. The basic reactionis shown in Equation 1 for copper and TCNQ. ##STR2## The directoxidation-reduction reaciton results in the forming of a highlymicrocrystalline film 2 directly on the copper or silver. The film 2,grown according to this technique, shows a metallic sheen and can begrown to a thickness of 10 μm in a matter of minutes. The thickness may,of course, vary depending on application. The thicker the film 2, thelonger the memory state lasts when the applied field is removed.

Once the polycrystalline film has been grown to the desired thickness,the growth process can be terminated by simply removing the metalsubstrate containing the organic layer film 2 from the acetonitrilesolution. This terminates the redox reaction. The two componentstructure, comprising the film 2 and the substrate electrode 4, isgently washed with additional acetonitrile to remove any excess neutralacceptor molecules and is then dried under a vacuum to remove any tracesof acetonitrile solvent. Elemental analysis performed on the bulk of thepolycrystalline films of Cu-TCNQ and Cu-TNAP in the "OFF" state revealsthat the metal/acceptor ratio is 1:1 in both salts. A three componentstructure is complete when a top metal electrode 6 of either aluminum orchromium is pressure contacted, evaporated, or sputtered directly on theorganic film 2.

It is believed that the solid-state, reversible electrochemical redoxreaction which results in switching in the present invention producesmixed valence species or complex salts. The salts can exist insolid-state equilibrium with the simple 1:1 salt as exemplified by theequation:

    [Cu.sup.+ (TCNQ..sup.-)].sub.n ⃡Cu.sub.x.sup.o +[Cu.sup.+ (TCNQ..sup.-)].sub.n-x +(TCNQ.sup.o).sub.x.

An ionic or a molecular displacement associated with this equiplibriumapparently explains the observed memory phenomena and the fact thatswitching, according to the present invention, is between only twostable resistive states.

Although the present invention has been described in detail with regardto copper complexed with TCNQ (and certain derivatives thereof) andTNAP, the present switching device 1 may also include films 2 of othercompositions (such as those listed above) and copper or silver complexedwith 2,3 dichloro 5,6 dicyano 1,4 benzoquinone (DDQ) which act subjectto the same phenomena and are within the redox potential spectrumdiscussed above. Of particular significance, the switching behavior andfabrication of the electron acceptor tetracyanoethylene (TCNE) issimilar to that of the above discussed TNAP, TCNQ, and TCNQ derivativeswhen complexed with copper or silver as previously described.

Various modifications, adaptations and alterations to the presentinvention are of course possible in light of the above teachings. Itshould therefore be understood at this time that within the scope of theappended claims, the invention may be practiced otherwise than wasspecifically described hereinabove. For example, a plurality of filmscan be grown onto a single or common substrate. Matrixes of threshold ormemory switches or both may be included on such a substrate by properselection of films.

What is claimed is:
 1. A field induced switching apparatus comprising:asolid-state organic semiconductor, said organic semiconductor comprisinga metal complexed with an organic electron acceptor selected from thegroup consisting of TCNQ, TCNQ derivatives, TCNE, TNAP and DDQ. forminga metal charge-transfer salt; an electric field applied to the surfaceof said organic semiconductor, said field being sufficient to induce asolid state reversible electrochemical reaction in said organicsemiconductor, causing at least a portion of said organic electronacceptor to switch from a first oxidation state to a second oxidationstate.
 2. The apparatus of claim 1, wherein said organic electronacceptor in said first oxidation state has at least one detectablydifferent property from said organic electron acceptor in said secondoxidation state.
 3. The apparatus of claim 2, wherein said firstoxidation state has an electrical impedance detectably different fromsaid second oxidation state.
 4. The apparatus of claim 2, wherein saidorganic electron acceptor is chosen from a class of weak electronacceptors having a low reduction potential thereby providing thresholdswitching from said first oxidation state to said second oxidationstate.
 5. The apparatus of claim 2, wherein said organic electronacceptor is chosen from a class of strong electron acceptors having ahigh reduction potential thereby providing memory switching from saidfirst oxidation state to said second oxidation state.
 6. The apparatusof claim 2, wherein said metal is selected from the group consisting ofsilver and copper.
 7. The apparatus of claim 2, wherein said organicelectron acceptor is TCNQ.
 8. The apparatus of claim 2, wherein saidorganic electron acceptor is TNAP.
 9. The apparatus of claim 2, whereinsaid organic electron acceptor is a TCNQ derivative.
 10. The apparatusof claim 2, wherein said organic electron acceptor is TCNQ(OMe).
 11. Theapparatus of claim 2, wherein said organic electron acceptor isTCNQ(OMe)₂.
 12. The apparatus of claim 2, wherein said organic electronacceptor is TCNQ(OMe)(OEt).
 13. The apparatus of claim 2, wherein saidorganic electron acceptor is TCNQ(OMe)(O-i-Pr).
 14. The apparatus ofclaim 2, wherein said organic electron acceptor is TCNQ(OMe)(O-i-Bu).15. The apparatus of claim 2, wherein said organic electron acceptor isTCNQ(OMe)(O-i-C₂ H₅).
 16. The apparatus of claim 2, wherein said organicelectron acceptor is TCNQ(OEt)(SMe).
 17. The apparatus of claim 2,wherein said organic electron acceptor is TCNQCl.
 18. The apparatus ofclaim 2, wherein said organic electron acceptor is TCNQBr.
 19. Theapparatus of claim 2, wherein said organic electron acceptor isTCNQClMe.
 20. The apparatus of claim 2, wherein said organic electronacceptor is TCNQBrMe.
 21. The apparatus of claim 2, wherein said organicelectron acceptor is TCNQIMe.
 22. The apparatus of claim 2, wherein saidorganic electron acceptor is TCNQI.
 23. The apparatus of claim 2,wherein said organic electron acceptor is TCNQ(OMe)(OCH₃)₂.
 24. Theapparatus of claim 2, wherein said organic electron acceptor isTCNQ(CN)₂.
 25. The apparatus of claim 2, wherein said organic electronacceptor is TCNQ(Me).
 26. The apparatus of claim 2, wherein said organicelectron acceptor is TCNQ(Et).
 27. The apparatus of claim 2, whereinsaid organic electron acceptor is TCNQ(i-Pr).
 28. The apparatus of claim2, wherein said organic electron acceptor is TCNQ(i-Pr)₂.
 29. Theapparatus of claim 2, wherein said organic electron acceptor is TCNE.30. The apparatus of claim 2, wherein said organic electron acceptor isDDQ.